DE102021204086A1 - heterodyne interferometer - Google Patents

Abstract

The invention relates to a heterodyne interferometer for determining a distance in a microlithographic projection exposure system. The heterodyne interferometer according to the invention has interferometer optics (8), a detector (7) for detecting light from a light beam that has passed through the interferometer optics (8), an optical fiber (6) designed as a gradient index fiber for transmitting the light from the interferometer optics (8) to the detector (7) and coupling optics (5) for adapting the diameter and the numerical aperture of the light beam to the optical fiber (6).

Description

The invention relates to a heterodyne interferometer.

In a heterodyne interferometer, interferometric measurements are performed with light having at least two different frequencies. The superimposition of the two frequencies creates a beat, the frequency of which is usually significantly lower than the two frequencies of light, but still represents a relatively high-frequency signal in the megahertz range, so that the noise component in the measurement can be kept low. Very precise measurements, for example very precise distance measurements, can thus be carried out with a heterodyne interferometer. Such distance measurements can be used, for example, to determine the position of a mirror or another optical element in a microlithographic projection exposure system, in particular EUV lithography. The mirror or other optical element can be moved to a desired position on the basis of the determined values for the position. In the case of a distance measurement in a microlithographic projection exposure system, the interferometer optics can be arranged, for example, within an illumination system or within a projection system and the detector can be arranged outside of the illumination system or projection system. The optical signal can then be transmitted from the interferometer optics to the detector by means of a light guide, in particular by means of an optical fiber.

That's it US4784489B a receiver for a dual frequency laser interferometer is known in which a detector photodiode is connected via an optical fiber to a portion of the receiver into which light from a dual frequency interferometer is fed.

Particularly in the case of a projection exposure system for EUV lithography, it can happen that the optical fiber used for forwarding the interference signals is several meters to several tens of meters long. If a conventional step-index multimode fiber is used as the optical fiber, the mode dispersion within this optical fiber can lead to very different propagation times of the signal for different fiber modes. External mechanical movements and vibrations can affect local bending radii of the optical fiber and cause mechanical stresses in the optical fibers, which in turn can lead to a change in the mode distribution within the optical fiber. This results in runtime variations that change over time, which are translated directly into phase and position errors with the split frequency of the heterodyne interferometer. All of these effects have a negative effect on the accuracy that can be achieved when measuring distances with the heterodyne interferometer.

The object of the invention is to use a heterodyne interferometer to achieve the highest possible measurement accuracy, in particular in a distance measurement in a microlithographic projection system and/or in particular in a spatially separate arrangement of interferometer optics and detector.

This object is achieved by the combination of features of claim 1.

The heterodyne interferometer according to the invention for determining a distance in a microlithographic projection exposure system has interferometer optics, a detector for detecting light from a light beam that has passed the interferometer optics, an optical fiber designed as a gradient index fiber for transmitting the light from the interferometer optics to the detector and coupling optics for adapting the diameter and the numerical aperture of the light beam to the optical fiber.

The heterodyne interferometer according to the invention has the advantage that it enables high measurement accuracy. The optical fiber used has a comparatively low mode dispersion, so that there are only small propagation time differences. This reduces the effects of vibrations, temperature effects and other environmental influences on the measurement accuracy. This enables the use of a relatively long optical fiber and thus a spatially separate arrangement of the interferometer optics and the detector without impairing the measurement accuracy to an impermissible extent. Accordingly, the heterodyne interferometer according to the invention is also particularly suitable for EUV lithography. The transit time variation can be adapted to the respective requirements by optimizing the refractive index profile of the optical fiber designed as a gradient index fiber.

Furthermore, the invention relates to a heterodyne interferometer for determining a distance in a microlithographic projection exposure system, with interferometer optics, a detector for detecting light of a light bundle that has passed the interferometer optics, an optical fiber for transmitting the light from the interferometer optics to the detector, the optical fiber has a mode dispersion of at most 1 ps/m and coupling optics to adjust the diameter serers and the numerical aperture of the light beam to the optical fiber.

The diameter of the light bundle when impinging on the optical fiber can essentially match the diameter of the fundamental mode of the optical fiber. This has the advantage that only a few modes are excited in the optical fiber and therefore only small transit time differences occur.

The numerical aperture of the light bundle when impinging on the optical fiber can essentially correspond to the numerical aperture of the optical fiber. As a result, the light losses can be kept low.

The coupling optics can have a telescope optics, for example. This enables the diameter of the light beam to be very well matched to the optical fiber.

A lens may be placed at an end of the optical fiber such that the lens touches the optical fiber. This enables efficient light coupling into the optical fiber. At least one end of the optical fiber may have an anti-reflective coating. Additionally or alternatively, at least one end of the optical fiber can have an angle deviating from 90° with the direction of the longitudinal extent of the optical fiber in a region of the optical fiber directly adjacent to the end. Both measures avoid or reduce disturbing back reflections at the fiber ends and associated etalon effects as well as disturbances in the propagation time characteristics.

The refractive index and the numerical aperture of the optical fiber can be adapted to the coupling optics by doping. This allows the use of an existing coupling optics.

The interferometer optics can have a back reflector, which is arranged on an optical element of the projection exposure system. This enables a distance measurement of the optical element.

The optical fiber can be in the form of a photonic crystal fiber. It is also possible for the optical fiber to be in the form of a single-mode fiber. This has the advantage that the propagation time properties of the light signal are hardly affected and a very high measurement accuracy can be achieved.

The invention further relates to an illumination system for microlithography with a heterodyne interferometer according to one of the preceding claims.

The invention also relates to a projection system for microlithography with a heterodyne interferometer according to the invention.

Finally, the invention relates to a projection exposure system for microlithography with a heterodyne interferometer according to the invention.

The invention also relates to an illumination system for microlithography, a projection system for microlithography and a projection exposure system for microlithography, each of which has the measuring system according to the invention.

The invention is explained in more detail below with reference to the exemplary embodiments illustrated in the drawing.

• 1 ein Ausführungsbeispiel eines erfindungsgemäßen Heterodyninterferometers in einer schematischen Darstellung,
• 2 einen Querschnitt und ein Brechungsindexprofil einer optischen Faser, die ein Stufenindex-Profil aufweist, in einer schematischen Darstellung,
• 3 einen Querschnitt und ein Brechungsindexprofil einer optischen Faser, die ein Gradientenindex-Profil aufweist, in einer schematischen Darstellung,
• 4 ein Ausführungsbeispiel der Einkoppeloptik in einer schematischen Darstellung,
• 5 eine mögliche Ausgestaltung der Einkoppelung in eine optische Faser, die als Singlemode-Faser ausgebildet ist, in einer schematischen Darstellung.

Show it

• 1 an embodiment of a heterodyne interferometer according to the invention in a schematic representation,
• 2 a cross section and a refractive index profile of an optical fiber having a step index profile in a schematic representation,
• 3 a cross section and a refractive index profile of an optical fiber having a graded index profile in a schematic representation,
• 4 an embodiment of the coupling optics in a schematic representation,
• 5 a possible embodiment of the coupling into an optical fiber, which is designed as a single-mode fiber, in a schematic representation.

1 shows an embodiment of a heterodyne interferometer according to the invention in a schematic representation.

The heterodyne interferometer has a heterodyne laser 1 , a polarization beam splitter 2 , a first back reflector 3 , a second back reflector 4 , coupling optics 5 , an optical fiber 6 and a detector 7 . The polarization beam splitter 2 and the two rear reflectors 3, 4 are collectively referred to as interferometer optics 8 in the following.

Furthermore are in 1 a housing 9 and an optical element 10 of a microlithographic projection system are also shown. The projection system can contain further optical elements who in 1 are not shown and is part of a projection exposure system of microlithography, in particular EUV microlithography, which has other components not shown in the figures, such as a light source, an illumination system, a reticle stage, a wafer stage, etc.

The interferometer optics 8, the coupling optics 5 and the optical element 10 are arranged within the housing 9 of the projection system. The heterodyne laser 1 and the detector 7 are arranged outside the housing 9 of the projection system. The optical fiber 6 is arranged partly inside and partly outside the housing 9 of the projection system.

The heterodyne laser 1 does not necessarily have to be assigned to the heterodyne interferometer, but can also be viewed as a separate component. The wavelength of the laser radiation generated by the heterodyne laser 1, which is 1 shown as a dashed line and whose direction of propagation is indicated by arrows, is 633 nm, for example. This laser radiation has two components that differ both in terms of their frequency and their state of polarization. A first component of the laser radiation has a frequency f ₁ and a second component of the laser radiation has a frequency f ₂ . The frequency difference between frequencies f ₁ and f ₂ is 10 MHz. Compared to the frequencies f ₁ and f ₂ themselves, which are each several hundred terahertz, the frequency difference is extremely small, so that both components have an almost identical wavelength of about 633 nm.

In the exemplary embodiment shown, both components of the laser radiation are linearly polarized, with the directions of polarization of the two components being perpendicular to one another. The direction of polarization of the first component of the laser radiation runs parallel to the plane of the drawing and is in 1 represented by a dash. The direction of polarization of the second component of the laser radiation runs perpendicular to the plane of the drawing and is in 1 represented by a point.

The laser radiation generated by the heterodyne laser 1 impinges on the polarization beam splitter 2, which is arranged inside the housing 9 of the projection system. When it strikes the polarization beam splitter 2, only one light bundle is present, which contains both components of the laser radiation. The polarization beam splitter 2 allows the first component of the laser radiation to pass without deflection, so that it hits the first back reflector 3, which is attached to the optical element 10 and thus moves in an identical manner to the optical element 10 with regard to at least one degree of freedom of movement. The first The component of the laser radiation is reflected back in a parallel offset manner by the first rear reflector 3 and hits the polarization beam splitter 2 again, passes through it without being deflected and finally hits the coupling optics 5.

The second component of the laser radiation impinging on the polarization beam splitter 2 is deflected by 90° by the polarization beam splitter 2 and then impinges on the second back reflector 4, which is stationary and the polarization beam splitter 2 is directly adjacent. In particular, the second back reflector 4 can be attached to the polarization beam splitter 2 . The second back reflector 4 reflects the second component of the laser radiation back in a parallel offset manner, so that it hits the polarization beam splitter 2 again. When passing through the polarization beam splitter 2 again, the second component of the laser radiation is again deflected by 90° and then hits the coupling optics 5. Both the impact area and the impact direction match the first component of the laser radiation.

The laser radiation is thus split by the polarization beam splitter 2 into a first light bundle, which contains the first component of the laser radiation, and into a second light bundle, which contains the second component of the laser radiation. The first light bundle and the second light bundle cover different optical paths through the interferometer optics 8 and, after passing through the interferometer optics 8, are combined again to form a common light bundle. The difference in the optical paths covered by the two light beams when passing through the interferometer optics 8 depends on the distance between the first back reflector 3 and the polarization beam splitter 2 and thus on the position of the optical element 10 .

The light of the combined light beam is coupled into the optical fiber 6 with the aid of the in-coupling optics 5 and is transmitted from the optical fiber 6 to the detector 7 . The in-coupling optics 5, the in-coupling process and the formation of the optical fiber 6 are described in more detail below.

The optical fiber 6 is a multimode fiber, so that when the light is coupled in, several modes are excited in the optical fiber 6 and the light signal is thus transported through the optical fiber 6 by means of several modes, each of which contains a portion of the transport light signals. The optical fiber 6 usually has a length of about 5 m to 30 m. A mode dispersion in the optical fiber 6 means that there are transit time differences in the parts of the light signal that are transmitted by different modes, which ultimately limit the achievable measurement accuracy. This is based on the 2 and 3 explained in more detail. After passing through the optical fiber 6, the light signal is detected by the detector 7 and used to determine the distance between the polarization beam splitter 2 and the first back reflector 3. From this, in turn, the position, more precisely a position coordinate, of the optical element 10 can be determined and used for position control of the optical element 10 . To detect the light signal, the detector 7 can have one or more photodiodes.

2 Fig. 12 shows a cross section and a refractive index profile of an optical fiber 6 having a step index profile in a schematic representation. In the upper part of the 2 the optical fiber 6 is shown in cross section. In the lower part of 2 the associated refractive index profile is shown, in which the refractive index n of the material of the optical fiber 6 is plotted against the spatial coordinate x. The optical fiber 6 has a cylindrical core 11 and a hollow-cylindrical cladding 12 which encloses the core 11 . The core 11 of the optical fiber 6 has a spatially constant refractive index n _core that is greater than a spatially constant refractive index n _cladding of the cladding 12 of the optical fiber 6 Core 11 of optical fiber 6 to a jump in refractive index n from the low value n _cladding of cladding 12 to the high value n _core of core 11. The difference in refractive index n between core 11 and cladding 12 of optical fiber 6 allows light to be guided within of the core 11, since the light is reflected in the radial region of the transition from the core 11 to the cladding 12 as long as the critical angle of total reflection is not exceeded.

At the in 2 The optical fiber 6 shown is a multimode fiber in which several modes can propagate at the same time. The core 11 of the optical fiber 6 can have a diameter of approximately 50 μm. Then the mode dispersion in the optical fiber 6 is about 10 to 20 ps/m. If an optical fiber 6 having a core 11 with a larger diameter is used, the mode dispersion can also be 100 ps/m or more.

For a mode dispersion of 10 ps/m and a length of the optical fiber 6 of 20 m, there is a transit time difference Δt between the modes of 200 ps. With a split frequency f _s = 100 MHz, a measuring wavelength λ = 633 nm and a 4-fold passage of the laser radiation through the interferometer optics 8, this transit time difference δt results in a position error δz of up to: $$\delta \mathrm{e.g}=\frac{1}{2\cdot 4}\cdot \lambda \cdot \delta t\cdot f_s=158pm$$

For values other than those mentioned above, the transit time difference δt caused by the mode dispersion can be estimated using the following formula: $$\delta t=\frac{L\cdot \mathrm{<figure-callout\; id="2"\; label="N\; A"\; filenames="DE102021204086A1\_0003.png"\; state="\{\{state\}\}">N</figure-callout>}{A}^{}{}^2}{2\cdot c\cdot n_{cO\mathrm{right}e}}$$

where L is the length, NA is the numerical aperture and n _core is the refractive index of the core 11 of the optical fiber 6 and c is the vacuum speed of light.

with the inside 2 With the optical fiber 6 shown, which has a step index profile, no substantial reduction in the propagation time difference δt and thus also in the position error δz can be achieved within the parameter space reasonably accessible within the scope of the invention. A position error δz in the range of 158 pm is usually too large for precise distance measurements in EUV microlithography, so that within the scope of the invention, a differently designed optical fiber 6, in particular an optical fiber 6 described in more detail below with a Gradient index profile is used.

3 Fig. 12 shows a cross section and a refractive index profile of an optical fiber 6 having a graded index profile in a schematic representation. The type of representation is analogous to 2 chosen.

In accordance with 2 also points out the in 3 The optical fiber 6 shown has a cylindrical core 11 which is surrounded by a hollow cylindrical cladding 12 and the core 11 of the optical fiber 6 has a refractive index n _core which is greater than a refractive index n _cladding of the cladding of the optical fiber 6. However, the refractive index n _core of the core 11 of the in 3 illustrated optical fiber 6 is not spatially constant, but decreases radially outward according to a predetermined function continuously. Thus, in the radial area of the transition from the cladding 12 to the core 11, there is no jump in the refractive index n from the low value n _cladding of the cladding 12 to the high value n _core of the core 11, but rather a continuous progression. For example, the refractive index n _core of the core 11 can decrease radially outwards according to a parabolic profile. Differently designed profiles are also possible and it is also possible to use a suitable one Profile to be determined by optimization taking into account the maximum permissible runtime difference δt.

A comparatively small number of modes are excited in an optical fiber 6 with a gradient index profile. In addition, the optical path length, i. H. the refractive index integrated over the path covered, is almost the same for all modes in an optical fiber 6 with a graded index profile given a suitable choice of the radial refractive index profile. Therefore, there is only a small mode dispersion and accordingly also a small delay time difference δt between the modes and thus a small position error δz.

with the inside 3 A mode dispersion of 1 ps/m or less can be achieved with the fiber 6 shown. This value corresponds to 10% of the mode dispersion of the in 2 illustrated optical fiber 6 having a step-index profile. Accordingly, the position error δz caused by the optical fiber 6 is reduced to 10% of that for 2 mentioned value, ie to approx. 16 pm. with the inside 3 The optical fiber 6 shown, which has a gradient index profile, can thus achieve a very high measurement accuracy.

A further reduction in the transit time difference Δt can be achieved by designing the way in which the laser radiation is coupled into the optical fiber 6 . It is favorable to design the in-coupling in such a way that as few modes as possible are excited in the optical fiber 6 . This is explained below using the 4 and 5 explained in more detail.

4 shows an embodiment of the coupling optics 5 in a schematic representation.

In the 4 The coupling optics 5 shown have a first lens 13 and a second lens 14 . The first lens 13 has a positive refractive power and is arranged or formed directly at an end of the optical fiber 6, ie the first lens 13 touches the optical fiber 6 and there is no air gap between the first lens 13 and the optical fiber 6. In particular For example, the first lens 13 may be formed integrally with the optical fiber 6. In particular, the first lens 13 can only extend over the diameter range of the core 11 of the optical fiber 6 in order to avoid coupling into the cladding 12 of the optical fiber 6 . The second lens 14 , which has a significantly larger diameter than the first lens 13 , is arranged at a distance from the first lens 13 . The second lens 14 also has a positive refractive power.

After passing through the interferometer optics 8, the light beam formed by the laser radiation, which 4 indicated by dashed lines, has a substantially larger diameter than the optical fiber 6, in particular than the core 11 of the optical fiber 6. The second lens 14 and the first lens 13 together form a telescope that adapts the diameter of the light beam to the optical fiber 6 . In particular, the diameter of the light beam is adjusted so that it essentially corresponds to the diameter of the fundamental mode of the optical fiber 6 . When dimensioning this telescope, the numerical aperture NA of the optical fiber 6 is also taken into account and the light bundle is adapted to this numerical aperture NA. Conversely, it is also possible to adapt the refractive index n and the numerical aperture NA of the optical fiber 6 to the telescope or other coupling optics 5 by doping the optical fiber 6 . Thus, after passing through the coupling optics 5, the laser radiation hits the optical fiber 6 as a light beam with a suitable diameter and a suitable numerical aperture. The consequence of this is that only a few modes or even only the fundamental mode are excited in the optical fiber 6 or will.

A single lens with a long focal length can also take the place of the telescope formed by the second lens 14 and the first lens 13 if the installation space permits.

The numerical aperture of the laser radiation can be adapted to the optical fiber 6 by placing the beam waist of an ideally Gaussian laser beam in the plane of the end of the optical fiber 6 .

There is also the possibility of including an optical fiber 6, which is designed as a single-mode fiber, in the signal transmission between the interferometer optics 8 and the detector 7. However, direct coupling of the laser radiation after it has passed through the interferometer optics 8 is generally unfavorable, since this is associated with a very high loss of light. The loss of light can be reduced by a gradual coupling. This is based on 5 explained.

5 shows a possible embodiment of the coupling into an optical fiber 6, which is designed as a single-mode fiber, in a schematic representation.

In 5 a coupling fiber 15 is shown, which is designed as a multimode fiber and spliced onto a graded index lens 16 . At the gradient index lens 16 closes the opti cal fiber 6, which is designed as a single-mode fiber.

The laser radiation, which has previously passed through the interferometer optics 8, can be coupled into the coupling fiber 15 in the manner already described for coupling into a multimode fiber. After passing through the coupling fiber 15, the laser radiation enters the gradient index lens 16. The gradient index lens 16 has a course of the refractive index, which is indicated by dashed lines and which changes in the longitudinal direction of the gradient index lens 16 as it approaches the as a single mode -Fiber-trained optical fiber 6 constricts radially. As a result, a positive refractive power is generated in the longitudinal direction of the gradient index lens 16, which results in a reduction in the diameter of the light beam when passing through the gradient index lens 16. In this way, the diameter of the light beam is adapted to the very small diameter of the optical fiber 6 designed as a single-mode fiber without any significant loss of light, and coupling into the optical fiber 6 takes place.

In a modification of the in 5 In the illustrated embodiment, the graded index lens 16 is replaced by a tapered fiber, ie, a fiber having a cross-section that narrows along the length of the fiber.

In all of the exemplary embodiments, the end faces of the optical fiber 6 can be provided with an anti-reflection layer in order to avoid disruptive back reflections and associated etalon effects or additional disruption to the propagation time properties due to multiple passes. Additionally or alternatively, it is possible to bevel the end faces of the optical fiber 6.

Instead of being designed as a gradient index fiber, the optical fiber 6 can also be designed as a photonic crystal fiber.

Reference List

1

heterodyne laser

2

polarizing beam splitter

3

First rear reflector

4

Second rear reflector

5

coupling optics

6

optical fiber

7

detector

8th

interferometer optics

9

Housing

10

optical element

11

core

12

a coat

13

First lens

14

Second lens

15

launch fiber

16

gradient index lens

QUOTES INCLUDED IN DESCRIPTION

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Patent Literature Cited

• US4784489B [0003]

Claims (15)

Heterodyne interferometer for determining a distance in a microlithographic projection exposure system, with - an interferometer lens (8), - a detector (7) for detecting light of a light beam that has passed the interferometer optics (8), - an optical fiber (6) designed as a gradient index fiber for transmitting the light from the interferometer optics (8) to the detector (7) and - A coupling lens (5) for adapting the diameter and the numerical aperture of the light beam to the optical fiber (6). Heterodyne interferometer for determining a distance in a microlithographic projection exposure system, with - an interferometer lens (8), - a detector (7) for detecting light of a light beam that has passed the interferometer optics (8), - an optical fiber (6) for transmitting the light from the interferometer optics (8) to the detector (7), the optical fiber (6) having a mode dispersion of at most 1 ps/m and - A coupling lens (5) for adapting the diameter and the numerical aperture of the light beam to the optical fiber (6). Heterodyne interferometer according to one of the preceding claims, in which the diameter of the light beam when impinging on the optical fiber (6) essentially corresponds to the diameter of the fundamental mode of the optical fiber (6). Heterodyne interferometer according to one of the preceding claims, in which the numerical aperture of the light beam when impinging on the optical fiber (6) essentially corresponds to the numerical aperture of the optical fiber (6). Heterodyne interferometer according to one of the preceding claims, in which the in-coupling optics (5) have telescopic optics. Heterodyne interferometer according to one of the preceding claims, wherein a lens (13) is arranged at one end of the optical fiber (6) in such a way that the lens (13) touches the optical fiber (6). A heterodyne interferometer as claimed in any preceding claim, wherein at least one end of the optical fiber (6) has an antireflection coating. A heterodyne interferometer as claimed in any preceding claim, wherein at least one end of the optical fiber (6) is at an angle other than 90° with the direction of elongation of the optical fiber (6) in a region of the optical fiber (6) immediately adjacent the end. Heterodyne interferometer according to one of the preceding claims, in which the refractive index and the numerical aperture of the optical fiber (6) are adapted to the coupling optics (5) by doping. Heterodyne interferometer according to one of the preceding claims, in which the interferometer optics (8) have a back reflector (3) which is arranged on an optical element (10) of the projection exposure system. Heterodyne interferometer according to one of claims 2 until 10 , wherein the optical fiber (6) is designed as a photonic crystal fiber. Heterodyne interferometer according to one of claims 2 until 10 , wherein the optical fiber (6) is designed as a single-mode fiber. Microlithography illumination system with a heterodyne interferometer according to one of the preceding claims. Microlithographic projection system with a heterodyne interferometer according to one of Claims 1 until 12 . Microlithographic projection exposure system with a heterodyne interferometer according to one of Claims 1 until 12 .

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